For achieving the most constant possible pressure in the body cavity during the operation, there may be used an insufflation device such as that disclosed in DE-B-30 00 218, for example. In this case the gas is supplied to the body cavity from a gas container by way of an intermediate container, a pressure reducer, a stop valve, a pressure measuring instrument and a flow display device and by way of an admission pipeline and a cannula penetrating the integument.
Where a coagulation instrument is passed through the endoscope, for coagulating tissue, blood vessels and the like, its use causes smoke and steam to be produced in the body cavity, obscuring the operator's view of the operation. In order to avoid this disadvantage, a smoke filter circuit, as described in DE-B-33 29 784, for example, may be employed. This consists of a pump, a conduit and a filter. Insufflation gas mixed with smoke and steam is drawn off from the body cavity by the pump, is led through the conduit to the smoke filter, is cleaned in the filter and is finally returned to the body cavity.
In an insufflation procedure it is now usual for a nominal pressure for the body cavity to be set on an insufflation gas supply device and the body cavity to be filled with the insufflation gas until said nominal value has been reached, so as to keep the body cavity pressure as constant as possible during the operation. The constancy of said pressure, may, however, be disturbed by loss of gas caused, for example, by resorption of gas into the body, or by gas leaking at the entry point of the cannula into the body cavity. The resulting loss of pressure in the body cavity must be compensated for during the operation, by producing a regular slight inflow of gas into said cavity. The reference value for regulating such inflow is the nominal value which was set initially. The inflow must be controlled to achieve said nominal value, and to do this, the actual value of the pressure in the body cavity must be measured. Such measurement is generally effected at the pipe by means of which the gas is introduced into the body cavity, in order to avoid the need for making a second puncture therein.
The instruments and pipes normally used in endoscopy, are of such small cross section, that a difference in pressure occurs which distorts the measurement of said actual pressure so that the compensating gas inflow is difficult to control.
Such distortion is increased where a waste gas filter circuit is in operation for cleaning the insufflation gas as described above, since the output of the pump of the waste gas filter circuit is considerably higher than the output of the insufflation gas supply device when operated to deliver small amounts of gas in order to compensate for gas losses.
If the pipe of the waste gas filter circuit is connected to a pipe common to it, and to the gas supply device, in order to avoid the need for providing a further entry point in the body cavity, the said pump will, when in operation, generate such high dynamic pressure in the common pipe that, the measurement of the actual value of the pressure in the body cavity will be falsified so that the actual value will generally appear to be much greater that it really is.
It has been found that the said dynamic pressure may be many times greater than the desired pressure in the body cavity.
Such falsification of the measurement of said actual value prevents adjustment of the pressure in the body cavity until the sum of the actual value of the body cavity pressure and the said dynamic pressure falls below the nominal value of the pressure in the body cavity, whereby the body cavity may be completely emptied when gas has been lost, without adjustment of the pressure therein.